There are many fluid handling systems wherein the upstream fluid levels or pressures vary and it is desirable to passively control the rate of release with a flow control system which requires no human intervention or external energy source to activate. Examples of these systems include storm water detention ponds, storage reservoirs, holding tanks, surge tanks and the like. In general, these systems receive varying rates of fluid flow, which at times may exceed the desirable release rate or range of release rates as the case may be. When the inflow rate to these systems exceeds the release rate, a volume of fluid is stored and in response, the upstream fluid level rises. Conversely, when the release rate exceeds the inflow rate, the stored volume of fluid is released and the upstream fluid level falls.
Historically, the release rate from these type systems has been passively controlled with weirs and orifices, both of which produce flow rates which increase exponentially as the upstream fluid level rises. When inflow rates dramatically exceed the desirable outflow rate, this characteristic often results in a system wherein a large volume of storage is required to control the fluid at or below the level which does not produce a release in excess of the desired rate or range of rates. Since the costs of land acquisition, engineering, construction and transportation associated with creating large volumes of storage can be quite expensive, it is advantageous to design these systems with as little volume as possible. This objective is accomplished when the system design accommodates release at the desired rate for the maximum amount of time, as is depicted in FIG. 3 of U.S. Pat. No. 7,052,206 to Mastromonaco.
The prior art is replete with a variety of possible solutions to this design problem. One such example is disclosed in U.S. Pat. No. 7,125,200 to Fulton wherein he describes a flow control device for a holding pond consisting of a buoyant flow control module housing an orifice within an interior chamber that is maintained at a predetermined depth below the water surface. Fluid discharged through the orifice is conveyed to the outlet through a bellow, an accordion like conduit which facilitates vertical motion of the buoyant flow control module. Another such example is disclosed in U.S. Pat. No. 6,997,644 to Fleeger, wherein he describes a floating weir assembly for incorporation into a detention vessel. The floating weir assembly is supported by a buoyant means which maintains the weir opening at a predetermined depth below the water surface. In order to facilitate vertical motion, fluid passing over the weir is conveyed to the outlet by means of a hose which has greater capacity than the discharge produced by the weir. In U.S. Pat. No. 6,474,361 to Poppe a floating weir assembly is described which is ballasted with a flowable medium such as liquid or a solid particulate matter like sand to maintain the submergence of the assembly at predetermined depth below the fluid surface. Similar to Fleeger's disclosure, the preferred means to convey discharge to the outlet is a flexible hose. In U.S. Pat. No. 7,762,741 to Moody, co-inventor of the present disclosure, a flow control system for incorporation into a detention pond or surge tank is described whereby a moving riser, made buoyant and supported by at least one float is suspended within a stationary riser such that the opening to the moving riser is maintained at a fixed and predetermined depth below the fluid surface. Discharge through and around the moving riser is conveyed to the outlet through the stationary riser. Each of the foregoing examples utilize buoyancy to maintain the opening of a fluid passage at a fixed and predetermined depth below the upstream fluid surface.
Other proposed solutions have used buoyancy to effect changes in the area of a fluid passage. One such example is disclosed in U.S. Pat. No. 8,043,026 to Moody, co-inventor of the present disclosure, wherein a flow control system for incorporation into a detention pond or surge tank is described. The flow control system comprises a tapered plunger, suspended from at least one float, and the tapered plunger is located within and at the bottom edge of a vertically oriented tube which is fixed in a stationary position. Fluid from the upstream reservoir enters the tube from the upper end and its flow is restricted at the bottom end. As the upstream fluid level changes, at least one float moves the tapered plunger such that the area of the fluid passage between the inside, bottom edge of the stationary tube and the outside edge of the tapered plunger is reduced when the fluid level rises and conversely, increases as the fluid level falls. In the preferred embodiment, the taper of the plunger is formed such that the change in the area of the fluid passage maintains the flow rate at a constant rate.
Other solutions have sought to change the area of the fluid passage by the use of pressure rather than buoyancy. In U.S. Pat. No. 5,887,613 to Steinhardt a flow control system is disclosed whereby fluid pressure acts against a “form-changeable member” with a hollow interior which is connected to a pressure different from the pressure outside of the “form-changeable member”. The “form-changeable member” is biased with a spring against a bracket which supports a gate over the fluid passage. As the fluid level in the upstream reservoir rises, the pressure acting on the “form-changeable member” in turn rises, acting against the spring and reducing the area of the fluid passage. In the preferred embodiment, the bias of the spring and the geometry of the gate are designed such that the flow rate through the fluid passage remains constant as the upstream fluid levels both rise and fall.
Although all of these disclosures are passively operated and can theoretically control the rate of fluid release at a constant or nearly constant rate, they all rely on floats, springs and/or flexible conduits operating in conditions wherein the pressure on the outside of the conduit is higher than the pressure inside the conduit. These features have proven to be problematic for a number of reasons. Floats which are hollow can rupture and floats which are solid can absorb water over time which increases their density and reduces their net buoyancy. Springs may suffer from decreasing bias over time due to strain and repeated cyclical motion. Metallic springs can often corrode due to a variety of elemental exposures. Flexible conduits such as bellows and hoses, operating in conditions where the pressure outside of the conduit is greater than the pressure inside of the conduit, may collapse from the effects of excess hydrostatic pressure, often at depths much less than the design depth of the storage reservoir in which they are immersed.
Accordingly, there is a need for a flow control system that does not rely on floats, biasing members such as springs, or flexible conduits which are immersed in the fluid reservoir from which the flow control system is intended to control the release.